Male aggression in the context of sexual competition is ubiquitous in the animal kingdom. Humans prove to be no exception to this generalization. Sexual competition and its attendent emotions (e.g., sexual jealousy) appear to represent a common motive for spouse abuse, physical assault, and homicide in our own species. The overall objective of this research project is to investigate the neurobiological basis of primate male aggression in the context of sexual competition for females. The four specific aims for this project are: 1) use Positron Emission Tomography to characterize the neural response of male rhesus monkeys to a competitive mating context that frequently provokes aggression in socially housed animals, 2) characterize the hormonal response of adult male and female rhesus monkeys to a competitive mating context that frequently provokes aggression in social groups, 3) examine the correlation of brain activity in various anatomical regions of interest with both hormone levels and measures of agonistic behavior, 4) describe the effects of the anti-androgen flutamide on the pattern of brain glucose metabolism, the level of agonistic responding, and the hormone levels observed in aims 1 and 2. Ten groups of three adult rhesus monkeys will be newly formed for this study. Each group will consist of an ovariectomized, estrogen-treated female and two adult males. PET images of regional cerebral glucose metabolism, a corollary of synaptic activity, will be acquired from the male that emerges as dominant in each group (defined by his mating success with the female) in two different conditions. In one condition, the isolated dominant male will observe the lower-ranking male and female mate in a nearby cage. In socially housed animals, this stimulus frequently provokes an attack by the dominant male on the female of the mating pair. In the second condition (the control condition), the dominant male will again be confronted with the subordinate male-female pair, but this time after they have already mated out of his view (during the male's post-ejaculatory refractory period). Thus, in the control condition, the dominant male will not observe the pair mating. Five of the dominant males will be treated with an anti-androgen prior to experimental testing, and the other five will be untreated. For the five untreated animals, images from the control condition will be subtracted from images from the mating condition to identify brain regions that show increased synaptic activity in response to this aggression-arousing stimulus. The same analysis will be conducted in the anti-androgen treated animals. The pattern of activation in the two groups of males will be compared to determine if any of the observed activations in the non-treated animals are androgen-dependent. Blood samples will also be collected from all three animals in each group following both experimental conditions. Samples will be assayed for LH, FSH, testosterone, ACTH, cortisol and NE. Hormone levels will be correlated with PET activity levels in various anatomical regions of interest and with measures of aggressive responding (i.e., threats) as recorded by a video camera. This will permit attempts to relate localized changes in brain activity to specific behavioral and endocrine correlates of aggression. Most of what is currently known about the neurobiological basis of aggressive behavior is based on research with rodents and felines. This study proposes to investigate this issue using an animal (rhesus monkeys) that is more closely related to humans in critical behavioral and neuroanatomic features. Ultimately, it is hoped that a thorough understanding of the neural substrates of aggressive behavior may lead to treatments that curtail its pathological expression in domestic violence, assault and homicide.